The present invention relates to a reduced pressure device, and more particularly to a reduced pressure device capable of realizing a ultrahigh vacuum and ultraclean process.
In recent years, techniques for realizing ultrahigh vacuum or techniques for supplying fixed gases into a vacuum chamber at a small flow rate to provide an ultraclean, reduced pressure atmosphere have become very important. Such techniques are widely used in the study of material characteristics, the formation of various films, and the manufacture of semiconductor devices with advances in high vacuum techniques. It is therefore desired to obtain a reduced pressure atmosphere, wherein the impurity particles and molecules are reduced to the smallest limit.
Taking a semiconductor device as an example, in order to enhance the degree of integration of an integrated circuit, the search for putting into practical use semiconductor devices which are composed of unit elements, their sizes being 1 .mu.m to submicron and even to 0.5 .mu.m or less, is intensively carried out as the size of unit elements decreases year by year.
To manufacture such semiconductor devices it is necessary to repeatedly carry out film formation processes and etching processes for those formed films according to fixed circuit patterns. Usually, such processes are performed by placing silicon wafers into a vacuum chamber which has been evacuated to an ultrahigh vacuum state or into a vacuum chamber at a reduced pressure atmosphere whereinto fixed gases are introduced. If impurities appear in the vacuum chamber during these processes, problems such as the deterioration of film quality and a reduction of the precision of fine processing will take place. This is the reason that un ultrahigh vacuum or an ultraclean reduced pressure atmosphere are required.
One of the most important reasons for hindering the realization of an ultrahigh vacuum and an ultraclean atmosphere has been the release of gases from the surface of stainless steel, which material is widely used in chambers and piping systems. In particular, moisture adsorbed in the surfaces which will be released at a vacuum or reduced pressure atmosphere acts as the greatest source of contamination.
FIG. 8 shows, for a prior art device, the relationship of the total gas leakage amount of a system including a gas piping system and a reaction chamber (the total amount of gases released from the internal surfaces of the piping system and the reaction chamber and the gases due to leakage into the chamber from the exterior) and the contamination of gas at different flow rates.
In order to realize a highly precise process, there is a tendency to adopt smaller and smaller gas flow rates. For example, the selection of a flow rate of 10 cc/min or less has become generally accepted. If the flow rate of 10 cc/min is used and the system total leakage is in a range of 10.sup.-3 .about.10.sup.-6 Torr l/sec for a currently used prior art device, the purity of gas will reduce to 10 ppm.about.1%. Therefore such a process is far from being an ultraclean process.
The present invention comprises an ultraclean gas supply system and permits reducing the external leakage amount below the detectable limit of 1.times.10.sup.-11 Torr l/sec of existing detectors. However, because of internal leakage, i.e., the existence of released gas emanating from the internal stainless steel surfaces, the present invention fails to lower impurity concentrations in a reduced pressure atmosphere. In the case of stainless steel, the minimum amount of gas released from surfaces which are treated with prior art ultrahigh vacuum techniques is 1.times.10.sup.-11 Torr l/sec.multidot.cm.sup.2. By way of example, since the internal exposed surface area of a chamber is at least, 1 m.sup.2, the total leakage amount will be 1.times.10.sup.-7 Torr l/sec and gas with a purity of 1 ppm can be obtained only if the gas flow rate is 10 cc/min. Therefore it is evident that, if the gas flow rate is decreased still further, the purity will drop further.
In order to lower the degassing components released from the internal surface of chamber to the same level of 1.times.10.sup.-11 Torr l/sec.multidot.cm.sup.2, a surface treating technique for reducing the amount of gas from stainless steel is required.
Many gases of all kinds are employed in the semiconductor manufacturing process such as relatively stable general gases (O.sub.2, N.sub.2, Ar, H.sub.2, He) and other special gases with, respectively, strong reactivity, corrosivity and toxicity. The existence of moisture in a special gas atmosphere will cause a hydrolysis reaction which produces hydrochloric acid and fluoric acid, since BCl.sub.3 and BF.sub.3 etc., all having strong corrosivity, occur in special gases. Because of considerations of reactivity, corrosion-resistance, high strength, readiness of secondary processing, weldability and the ability to polish internal surfaces, stainless steel is usually used as the material for piping and chambers for dealing with these gases.
However, although the corrosion-resistance of stainless steel in an atmosphere of dry gas is very good, stainless steel will be easily corroded if it is placed in an atmosphere formed of chlorine or fluorine gases. Because of the above-mentioned facts it is necessary to treat stainless steel for corrosion-resistance after polishing the stainless steel surface. There have been several treatment methods such as Ni--W--P coating (method of cleanness coating). This method not only causes cracks, and easily forms pin holes but also has the disadvantage that the amount of moisture and solvent residue absorbed on the internal surfaces is great since the process adopts the method of wet coating. Moreover there are other methods such as passivation treatment for producing a thin oxide film on the surface of metal. Stainless steel can be passivated only by immersing it into a liquid containing an efficient oxidizer. The passivation treatment of stainless steel is usually carried out by immersing it into a solution of nitric acid. However, because this method is still a wet process there remains much residue of moisture and treatment solution in the pipes and chambers. Particularly chlorine and fluorine gases will cause severe damage to stainless steel being treated in this way.
Therefore although it is very important for an ultrahigh vacuum technique and semiconductor process to use chambers and gas supply systems constructed of stainless steel on which a passivation film is formed for occluding and adsorbing less moisture and which is capable of resisting damage causes by corrosive gases. Such a technique has not been developed in the prior art.
In view of the drawbacks of the prior art techniques it is an object of the present invention to provide a ultrahigh vacuum and ultra-clean reduced pressure device capable of reducing impurities resulting from released gas and having excellent corrosion-resistance.
Another object of the invention is to provide a reduced pressure device capable of performing self-cleaning and self-maintenance.